Better infos for dependent class parameter references

We sometimes create a dependent parameter reference p.X where
`p` is a path with a type that has a wildcard argument, e.g. `P[X >: L <: H].`
So far the denotation of `p.X` had as info the bounds with which `X` was
declared in `P`. Now it gets the actual parameter bounds instead.

Fixes #15652

#15652 started failing when tuple unpackings started to use `val`s instead of `def`s
in #14816. That caused dependent class parameter references to be created and uncovered
the problem.

Author: odersky

compiler/src/dotty/tools/dotc/core/Denotations.scala

@@ -1118,6 +1118,10 @@ object Denotations {
       if !owner.membersNeedAsSeenFrom(pre) && (!ownerIsPrefix || hasOriginalInfo)
          || symbol.is(NonMember)
       then this
+      else if symbol.isAllOf(ClassTypeParam) then
+        val arg = symbol.typeRef.argForParam(pre, widenAbstract = true)
+        if arg.exists then derivedSingleDenotation(symbol, arg.bounds, pre)
+        else derived(symbol.info)
       else derived(symbol.info)
     }
   }

compiler/src/dotty/tools/dotc/core/TypeOps.scala

@@ -426,8 +426,7 @@ object TypeOps:
     override def apply(tp: Type): Type =
       try
         tp match
-          case tp: TermRef
-          if toAvoid(tp) =>
+          case tp: TermRef if toAvoid(tp) =>
             tp.info.widenExpr.dealias match {
               case info: SingletonType => apply(info)
               case info => range(defn.NothingType, apply(info))

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -2453,10 +2453,12 @@ object Types {
       ctx.base.underlyingRecursions -= 1
 
     /** The argument corresponding to class type parameter `tparam` as seen from
-     *  prefix `pre`. Can produce a TypeBounds type in case prefix is an & or | type
-     *  and parameter is non-variant.
+     *  prefix `pre`. Can produce a TypeBounds type if `widenAbstract` is true,
+     *  or prefix is an & or | type and parameter is non-variant.
+     *  Otherwise, a typebounds argument is dropped and the original type parameter
+     *  reference is returned.
      */
-    def argForParam(pre: Type)(using Context): Type = {
+    def argForParam(pre: Type, widenAbstract: Boolean = false)(using Context): Type = {
       val tparam = symbol
       val cls = tparam.owner
       val base = pre.baseType(cls)
@@ -2468,7 +2470,7 @@ object Types {
           while (tparams.nonEmpty && args.nonEmpty) {
             if (tparams.head.eq(tparam))
               return args.head match {
-                case _: TypeBounds => TypeRef(pre, tparam)
+                case _: TypeBounds if !widenAbstract => TypeRef(pre, tparam)
                 case arg => arg
               }
             tparams = tparams.tail
@@ -5803,10 +5805,10 @@ object Types {
             // if H#T = U, then for any x in L..H, x.T =:= U,
             // hence we can replace with U under all variances
             reapply(alias.rewrapAnnots(tp1))
-          case tp: TypeBounds =>
+          case bounds: TypeBounds =>
             // If H#T = ? >: S <: U, then for any x in L..H, S <: x.T <: U,
             // hence we can replace with S..U under all variances
-            expandBounds(tp)
+            expandBounds(bounds)
           case info: SingletonType =>
             // if H#x: y.type, then for any x in L..H, x.type =:= y.type,
             // hence we can replace with y.type under all variances

tests/pos/i15652.scala

@@ -0,0 +1,12 @@
+trait Node
+type NodeParser[T] = Node => T
+
+def child(key: String): Option[Node] = ???
+
+def optionalOneOf[T](in: Map[String, NodeParser[? <: T]]): Option[T] =
+  val mappings = in flatMap { (key, parser) =>
+    child(key) map { node =>
+      key -> (() => parser(node))
+    }
+  }
+  mappings.headOption map (_._2())